B0 field inhomogeneity estimation using internal phase maps from long single echo time MRI acquisition

Abstract

A magnetic resonance (MR) image may be created from MR data by receiving the MR data, applying a transform to the MR data, where a result of the applying is an image space representation of the MR data, determining a wrapped phase map of the image space representation of the MR data, obtaining an unwrapped phase map based on the wrapped phase map, scaling the unwrapped phase map into a B0 field map, reconstructing the MR image based on the MR data, correcting the MR image based on the B0 field map, and outputting the MR image. The scaling may be free of accounting for effects on the MR data by artifact sources secondary to B0 field inhomogeneities.

We claim: 
     
       1. A method for creating a magnetic resonance (MR) image from MR data, the method comprising:
 receiving, by a processor, the MR data; 
 applying, by the processor, a transform to the MR data, wherein a result of the applying is an image space representation of the MR data; 
 applying, by the processor, a virtual coil combination method to the image space representation of the MR data, 
 determining, by the processor, a wrapped phase map of the image space representation of the MR data, wherein the wrapped phase map is determined based on the virtual coil combination method applied to the image space representation of the MR data; 
 obtaining, by the processor, an unwrapped phase map based on the wrapped phase map; 
 scaling, by the processor, the unwrapped phase map into a B0 field map, the scaling based on only the influence of B0 field inhomogeneities on phases in the MR data; 
 reconstructing, by the processor, the MR image based on the MR data; 
 correcting, by the processor, the MR image based on the B0 field map; and 
 outputting, by the processor, the MR image. 
 
     
     
       2. The method of  claim 1 , further comprising:
 determining, by the processor, a magnitude image based on the image space representation of the MR data. 
 
     
     
       3. The method of  claim 2 , further comprising:
 applying, by the processor, a sum of squares operation to the image space representation of the MR data, 
 wherein the determining of the magnitude image is based on the sum of squares operation applied to the image space representation of the MR data. 
 
     
     
       4. The method of  claim 2 , wherein the obtaining of the unwrapped phase map is based on the magnitude image. 
     
     
       5. The method of  claim 4 , further comprising:
 generating, by the processor, a mask image based on the magnitude image; and 
 applying, by the processor, the mask image to the B0 field map. 
 
     
     
       6. The method of  claim 1 , wherein the contribution of heating, motion, radio frequency pulse heterogeneities, coil sensitivities and eddy current, or a combination thereof to the phases in the MR data is unrepresented in the scaling. 
     
     
       7. The method of  claim 1 , wherein the MR data is received from a clinical acquisition, and
 wherein the correcting of the MR image is based on the B0 field map of only the clinical acquisition. 
 
     
     
       8. The method of  claim 1 , wherein the scaling of the unwrapped phase map is based on an approximation of the B0 field map as ΔB0=φ/ 2 π TE. 
     
     
       9. The method of  claim 1 , wherein the MR data is obtained from a non-Cartesian acquisition. 
     
     
       10. The method of  claim 1 , wherein the MR data is obtained with an echo time of at least 20 ms, a field strength of approximately 3 T, or a combination thereof. 
     
     
       11. The method of  claim 1 , further comprising:
 applying, by the processor, a low-pass filter to the B0 field map, the low-pass filter removing signals beyond a frequency domain of the B0 field inhomogeneities. 
 
     
     
       12. The method of  claim 1 , wherein the transform is a non-uniform transform. 
     
     
       13. A medical imaging system comprising:
 a magnetic resonance (MR) imager; 
 a processor; and 
 a memory, the memory storing instructions that, when executed, are operable to:
 receive MR data from the MR imager; 
 apply a transform to the MR data, wherein a result of the applying is an image space representation of the MR data; 
 determine a wrapped phase map of the image space representation of the MR data; 
 obtain an unwrapped phase map by solving a Poisson equation weighted by a magnitude of a previously estimated image; 
 scale the unwrapped phase map into a B0 field map, the scaling disregarding effects on the MR data by artifact sources secondary to B0 field inhomogeneities; 
 reconstruct the MR image based on the MR data; 
 correct the MR image based on the B0 field map; and 
 output the MR image. 
 
 
     
     
       14. The medical imaging system of  claim 13 , wherein the memory stores further instructions that, when executed, are operable to:
 apply a virtual coil combination method to the image space representation of the MR data, and 
 wherein the wrapped phase map is determined based on the virtual coil combination method applied to the image space representation of the MR data. 
 
     
     
       15. The medical imaging system of  claim 13 , wherein the memory stores further instructions that, when executed, are operable to:
 determine a magnitude image based on the image space representation of the MR data. 
 
     
     
       16. The medical imaging system of  claim 15 , wherein the memory stores further instructions that, when executed, are operable to:
 apply a sum of squares operation to the image space representation of the MR data, 
 wherein the magnitude image is determined based on the sum of squares operation applied to the image space representation of the MR data. 
 
     
     
       17. The medical imaging system of  claim 15 , wherein the unwrapped phase map is further obtained based on the magnitude image. 
     
     
       18. The medical imaging system of  claim 17 , wherein the memory stores further instructions that, when executed, are operable to:
 generate a mask image based on the magnitude image; and 
 apply the mask image to the B0 field map. 
 
     
     
       19. A non-transitory computer-readable medium storing processor-executable process steps, the process steps executable by a processor to cause a system to:
 receive clinical MR data from the MR imager; 
 apply a transform to the clinical MR data, wherein a result of the applying is an image space representation of the clinical MR data; 
 determine a wrapped phase map of the image space representation of the MR data by applying a virtual coil combination method to the image space representation of the MR data; 
 obtain an unwrapped phase map based on the wrapped phase map; 
 scale the unwrapped phase map into a B0 field map according to a linear relationship between B0 and echo time; 
 reconstruct the MR image based on the clinical MR data; 
 correct the MR image based on the B0 field map; and 
 output the MR image.